Mechanisms of deep brain stimulation in the Nucleus Accumbens are unknown. Electrical stimulation of this region is complex, activating all cells within the region of stimulation as well as excitatory and modulatory afferents. Therefore, it is necessary to specifically parse out effects on individual cells within the region. To determine the effect of stimulation on cells within the NAc, cell-type specific activation of MSNs was performed by using an adeno-associated virus expressing ChR2 in NAc D1-MSNs of Dynorphin-Cre/Tdtomato mice. D1- and D2-MSNs are known to release different peptides locally and at efferent projections. Current results indicate Substance P, exclusively expressed in D1-MSNs, may contribute to the behavioral effects of stimulation. To probe excitatory transmission, slices are prepared and whole-cell patch clamp is performed on D1-MSNs and D2-MSNs under fluorescent guidance. Excitatory input is electrically evoked prior to and following 50Hz (high frequency) optogenetic stimulation. Stimulation of D1-MSNs significantly depresses excitatory transmission on D1-MSNs, but potentiates excitatory transmission on D2-MSNs. Blocking the main receptor for Substance P, NK1 receptors, blocks potentiation on D2-MSNs and induces synaptic excitatory depression to the same degree as the stimulation-induced depression on D1-MSNs. Substance P (1uM) application mimics this potentiation effect on D2-MSNs in the NAc in a striatal dorsolateral to ventromedial gradient. These results suggest activation of NK1 receptors and Substance P release mediates the excitatory potentiation produced by stimulation. Furthermore, these effects are exclusively post-synaptic. Interestingly, using in situ hybridization, NK1 receptor expression is almost exclusively found in interneurons and all cholinergic interneurons express NK1 receptors. This suggests a disynaptic effect, potentially through cholinergic interneurons, may mediate the effect of Substance P on neuronal transmission. I hypothesize Substance P release from D1-MSNs increases the excitability of cholinergic interneurons and potentiates excitatory transmission on D2-MSNs through muscarinic 1 receptor mediated signaling. Future experiments will address whether acetylcholine plays a role in mediating the post-synaptic excitatory effect. In parallel, I am testing how various optogenetic stimulation protocols including frequency and pulse number and duration induce peptide release in the NAc and alter plasticity. Future experiments will examine how stimulation releases Substance P in vivo using microdialysis and in vivo imaging to determine if afferent stimulation can drive Substance P release. Furthermore, these stimulation parameters will be used to determine efficacy in treating pain-mediated opioid seeking.